The problem that I'm facing is that (although the network adapters are all Gigabit) my internet speeds seem to be limited on VM2.

I get between 40-50% of my internet bandwidth on VM2 but if I change the network adapter on VM2 to NAT (skipping opnsense and hooking onto the Dell Optiplex directly), I get 100% of my internet bandwidth (I tested the bandwidth by using wget to download a big iso file).

So the problem seems to be with some sort of throttling with the data going through opnsense.